BGP Path Selection

[aprasad11]

Hi,

I would like to know if i have two EBGP peering to two different AS number.

If i am advertising route 202.202.0.0/24 and 202.202.1.0/24 to ISP A and 202.202.0.0/23 to ISP B  which ISP will be chosen for return traffic.

In this case In the internet which route will be chosen,  the more specific route via ISP A who has a lot of prepends or route that has a better BGP attribute e.g better AS path etc.

[Jon Marshall · Accepted solution]

The most specific prefixes are always used.

So in your example  the BGP attributes don't come into it.

Jon

[aprasad11]

Thanks Jon.. Yes you are correct i had also simulated this ..

[Julio E. Moisa · Accepted solution]

Hi

It depends of the metric received, below you will find how BGP prefers a path over other:

https://www.noction.com/blog/bgp_bestpath_selection_algorithm

In the case of AS-Path attribute imagine you are receiving the route 1.1.1.1/32 via both ISP but with the following AS Path list:

ISP1  100  100 100
ISP 2  200  200 200 200 200

The router will prefer the path through ISP1, Comparing AS-Paths, the shortest AS-Path is always preferred. 

AS-Path also can be used to keep symmetric traffic. For example: Local-Preference (IN) could be used to know the best path to reach a destination and AS-Path (OUT) to prevent receive the traffic back through the backup path. 

Remember, if you are going to advertise a subnet to other BGP peer you need to include the specific (real) subnet mask, you have 2 - /24 subnets or one /23 subnet. 

Hope it is useful

:-)

[aprasad11]

Thanks For Your reply Julio,

Basically i have specific routes advertising via ISP A and a summarized aggregate advertising via ISP B. 

In my case the specific route is chosen first.

[Julio E. Moisa]

It was a pleasure my friend.

Have a good day!

:-)